Time-dependent variability of indoor radon concentrations in community centers based on high-resolution monitoring

Indoor radon is a leading source of natural radiation exposure, and its indoor concentrations vary substantially with building operation and ventilation conditions. Despite this, radon management in public facilities continues to rely on time-averaged measurements, which can obscure short-term accumulation episodes tied to occupancy and operational schedules. High-resolution continuous radon monitoring was conducted in 11 community service centers in Jeollanam-do, Korea, to characterize time-segmented concentration variability under real operational conditions. Hourly data were classified into four operational periods — working hours, extended working hours, non-working hours, and weekends — and analyzed using a linear mixed-effects model (LMM). Substantial inter-facility variability was observed across all concentration metrics. Radon concentrations were lowest during extended working hours and increased progressively during non-working hours and weekends. Variance decomposition attributed approximately 70% of total variability to facility-level differences (ICC ≈ 0.69), indicating that building-specific baseline characteristics exert a stronger influence on indoor radon exposure than operational time effects. Facility-specific temporal response patterns were further examined using continuous sensitivity indices alongside LMM-derived random intercepts, showing that temporal sensitivity and baseline radon level are not consistently related across facilities.